Photoelectric photometer



Oct 14, 19 47..v w. LIBEN EI'AL 'PBOTOELECTRIC Pnb'roumm Filed se t'. 26, 1944 :s Sheets-Sheet 1 Tab-3m P5050 02:15:40

. cwhu! IBM-mum U m J m .4 w t 2 in the low light level range.

Patented Oct. 14, 1947 PHOTOELECTRIO rno'rommm William Liben, New York, N. Y., and Franklin H.

Mackenzie, United States Army, Washington,

Application September 26, 1944, Serial No. 555.860

(Granted under the act of March 3, 1883, as amended April 30. 1928; 370 0. G. 757) 4 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without payment to us of any royalty thereon. The present invention relates to a photoelectric photometer and more particularly to a portable photometer capable oimeasuring light intensities of a wide range. I

An object of the invention-is to provide a portable photometer by means of which accu rate, direct readings-of light intensities may be be made in the field, for example to test the illumination oi! search lights or to determine illumination under blackout conditions. Another object of the invention is to provide a portable photometer combined-with means for calibrate ing the photometer without disturbing the circuit connections of the photometer. Other objects and advantages of the invention will appear from the following description and the accompanying drawings in which:

Figure 1 diagrammatically illustrates a circuit of a photometer incorporating the invention;

Figure 2 is a perspective view of a portable photometer incorporating the invention illustrating the arrangement of various elements for transportation;

Figure 3 is a partial view in perspective of the portable photometer with the photoelectric cell inserted in the calibrating socket;

Figure 4 is a partial perspective view of the portable photometer as assembled for use in the field;

Figure 5 is a vertical section of a portion of the photometer containing the calibrating socket with the photoelectric cell inserted as illustrated in Figure 3;

Figure 6 is a vertical section of the combined photoelectric cell housing and filter holder with the photoelectric cell inserted as illustrated in Figure 4;

Figure '7 illustrates a filter for use with the photometer; and

Figure 8 is a sectional view taken along the line 88 of Figure 7.

Referring to the drawings in detail, in the circuitillustrated in Fig. 1, there is a photoelectrlc cell! 0 connected to a single ampliiying stage. The photoelectric cell l0 should be a vacuum type cell having a linear output characteristic An example of a satisfactory type of photoelectric cell is one commonly designated by the trade as a 917. Other photoelectric cells may be used to increase sensitivity or to make the instrument more adaptable to lights of diilerent wave lengths.

nects with the central resistor 25''.

A cathode II and an anode l2 0! the photoelectric cell II are connected. respectively, to a grid l3 and a plate ll of an amplifying tube l5 through shielded leads I and I1. Soldered connections are made betweenthe leads i6 and i1 and the base of photoelectric cell ill to avoid variations in the electrical characteristics of the circuit during the use of the photometer.

The amplifying tube is illustrated is of the duplex triode type commonly known as a 1G6GT having a dual set of elements. There are two plates l4 and 23, two grids Band 33 and two filaments i3 and I9. A pair of triode amplifying tubes may housed in place of the 1G6GT tube. The filaments l3 and I! of the amplifying tube ii are connected to a 1.5 volt A battery 20. One side of each of the filaments l0 and I9 is connected through a switch 2| to'the positive side of the battery 20 and. the other side of each of the filaments is connected to the negative side of the battery 20 and to ground 22.

The plates l4 and 23 of the duplex triode tube l5 and the photoelectric cell ID are supplied with operating voltages from a volt B battery 24. The positive side of the battery 24 is connected to a variable center tap 25 of a potentiometer 26 shunted across the plates i4 and 23 of the amplifying tube. The potentiometer "consists of three 500 ohm resistors 23', 26" and 26" connected in series. The variable center tap 25 con- The resistors of the potentiometer 26 and the dual sets of elements in the amplifying tube form a bridge circuit that may be balanced by adjusting the center tap 25 of the potentiometer in a manner described hereinafter. A microammeter 28 having a scale reading from zero to fifty is also connected across the plates I4 and 23 of the tube IS. The sensitivity of the microammeter 28 may be regulated by adjustment of a 2500 ohm variable resistance 29 connected across the meter. Binding posts 30 and 3| are connected in series with the meter 28 and are normally shunted by a connecting link 32. If it is necessary to use a meter of greater sensitivity than the meter 28, the connecting link 32 is removed and the more sensitive meter is connected to the binding posts 30 and 3|.

A common biasing potential is applied to the grids l3 and 33 of the tube is by the drop across a 1000 ohm resistance 34 connecting-the grids i3 and 33 through 50 megohm grid leaks 35 and 36, respectively, to the negative side of the B battery 24 and to ground 22.

Under conditions where no light falls on the photoelectric cell l0 and consequently no potenj tial is impressed on the tube. Thevariable center tap 25 of potenti-" 3 ometer' 25" is adjusted so that the voltage drop 1 between contact tap 25 and plate l4 equals the voltagedrop between tap 25 andplate 23. In

; consequence of this adjustment no current will-- j .flow in the meter arm of the bridge circuit and reading of the microammeter 28 will bezero.

; When light falls on the photoelectric cell l8, s.

1 potential is applied to the grid l3 of the tube bridge between the anacoo grid l3 of the amplifier.

l5, thus changing the bias of the grid I8 and causing a corresponding change in the output of the plate I4 associated with it. This change in 1. output unbalances the bridge circuit and causes 1 current to flow through the microammeter 28.

. The values of the resistors given above were This arrangement permits use of a A calibrating bridge circuit is provided for calibrating the photoelectric cell L8 and the readings of the meter 28. This circuit is a Wheatstone bridge having a 3.8 volt flashlight bulb 31 and a 3.33 ohm resistance 38 connected in series as two arms of the bridge opposite two- 6 ohm resistances 89 and 48 connected in series 1 and forming the other two arms of the bridge. calibrating milliammeter 4| having a scale read-' ing from zero to five'is connected to the detector terminals of the bridge. The terminal of the two 6 ohm resistances 89 and 48 is connected through a .027 ohm resist- I ance 2l to the negative side of the A battery 1 28 and ground. The terminal of the bridge be 1 tween the 3.3.3 ohm resistance 88 and the flash- 1 light bulb 31 is connected through a one ohm that light from the lamp :1 will strike the photoelectric cell I8 when it is inserted in the socket.

'A contact to receive the anodeof the photoelectric cell l'8 is located in the base of the socket 58 and is wired to a terminal [54 located on the front panel 46. Connection between the terminal 54 and the lead I1 is made by a flexible lead 55 which extends from the adapter 49 as clearly shown in Fig. 5 of the drawings.

When the photoelectric cell I8 is placed in the calibrating socket 58, the light from the lamp 81, which has a fixed value of illumination, falls on the cell i8. This results in a reading on the microammeter 28. By adjusting the sensitivity resistance 29 the reading on the ammeter 28 may be made to correspond to the known value of the illumination from the lamp 31. As previously described the photoelectric cell l8 and the amplifying tube l5 are operated within the linear limits of theiroperating curves so that when the photoelectric cell has been calibrated in conjunction with the microammeter 28 for a known value of illumination, the ammeter may be read directly in foot candles for other values of illumination.

The provisions described above permit accurate calibration of the photometer immediately prior toits actual use and under the actualconditions to which the instrument is subjected withrheostat 42, a .035 ohm vernier rheostat 43 and 1 a switch 44 to the positive side of the A battery 28. By this arrangement the sensitivity of the 1 calibrating meter 4| may be raised so that a null point adjustment may be accomplished with a sensltivity'that' matches the photometer.

' By adjusting the resistances of therheostats 42 and 43 which are external to the bridge cir- 1 cuit itself, the resistance of the filament of the lamp 3'! may be varied to balance the bridge I circuit. The change in the resistance of the lamp filament is a function of its thermal coefficient so that when the resistances of the rheostats 42 and 43 are decreased, the resistance i of the lamp filament is increased. When the bridge circuit is balanced in the manner described, 'the illumination of the lamp 31'is fixed at a known value.

The components of the circuit described above with the exception of the photoelectric cellglll are assembled in a portable carrying case .45

A calibrating socket 58 is located in the carryhousing 5| has an aperture 52 extending through one side in line with the calibrating lamp 3! so out disturbing the electrical circuits -of the photometer and without establishing any additional circuits in the field. This is of great importance inasmuch as variations in the electrical circuits will greatly affect the operation of such a sensitive instrument.

After the photometer has been calibrated the 59 extends to the socket portion. The opening ing case 45. This socket 58 consists of a hous- 1 ing 5| to receive the photoelectric cell l8. The

59 permits light rays to strike the photoelectric cell when the cell is in the socket and the area of the opening 59 is approximately equal to the fiat plane area ofthe cathode II on which it is focused.

The elongated portion of the housing 55 is pro;

vided with two slots 68 and 6| in each of which.

glass Bland SS-andnmuntingthem in a Bakelite frame A set of three filters of various light transmitting powers is provided. Filter No. 1 includes a negative having a light transmission of ten per cent; filter No. 2 includes a negative having a light transmission of one per cent; and filter No. 3 includes two negatives each having a light transmission of one per cent so that the total light transmitting power of this filter is one hundredth of one per cent.

By using thev'arious filters and combinations thereof in conjunction with the photoelectric cell l8, the full scale reading of the microammeter may be made to read directly over a range from .05 to 50,000 foot candles. The full scale reading 01 the meter with various filters being as follows:

Full Scale Reading of Filter Meter Foot candle I ing system within said casing adapted to coact with said light-responsive cellto indicate the light response thereof, said system including a panel-mounted light meter and a pair of electrically shielded flexible cables extending from said panel, said cables constituting input and output leads for said system and terminating in said adapter, means in connection with said system for determining the intensity of a light source located at a distance from said casing, said means including a portable light receiver formed with a light opening and provided with means for seating said light-responsive cell in operative relation with respect to said light opening, and coacting means in connection with said light receiver, light-responsive cell and'adapter for establishing an electrical relation between said light-responsive device and said light-metering system through said adapter and the cables terminating therein, and means for calibrating said light meter in terms of units of illumination, said means including a light calibratin housing formed with a light opening, and a seat communicating with said opening and adapted,

to receive said light-responsive cell, a light source, means for adjusting said light source sothat its illuminating efiect corresponds to a predetermined unit of illumination, and coacting means between said calibrating housing, said light-responsive cell, and said adapter for establishing an electrical relation between said cell and said system through said adapter and the cables attached thereto.

2. In a photometric apparatus, the combination with a portable casing provided with an instrument panel, a photoelectric cell and adapter therefor, said adapter being removably associated e v 6 ing housing disposedadjacent for regulating said light source to provide an illuminating eiiect corresponding to a unit of illumination, means exterior of the casing for determining the intensity of a light source located at a distance from said casing and including a portable light receiver i'ormed with a light opening A brating housing and said light receiver.

with the casing; of a light metering system within said casing adapted to coact with said cell to indicate the response thereof,said system including a panel-mounted light meter and a pair of electrically shielded cables extending Irom said panel, said cables constituting input and output leads for said system and terminating at their free ends in said adapter, means within said casing for calibrating said light meter in terms 01 units 01' illumination, said means including a calibrat- 3. In a photometric apparatus, the combination with a portable casing provided with an instrument panel, a photoelectric cell and adapter therefor, said adapter being removably associated with the casing; of a light metering system within said casing adapted to coact with said cell to indicate the response thereof, said system including a panel-mounted light meter and a pair of electrically shielded cables extending from said panel, said cables constituting input and output leads for said system and terminating at their free ends in said adapter, means within said casingcfor calibrating said light meter in terms of units of iilumination, said means including a calibrating housing disposed adjacent said panel and formed with a seat adapted to receive said photoelectric cell, a light source within said housing in operative relation with respect to said cell, means for regulating said light source to provide the illuminating eifect of a unit of illumination, said means including a Wheatstone bridge network disposed within said casing and having said light source in one leg thereof, a panel-mounted calibrating meter across said bridge, a source of electrical energy, and means for balancing said bridge, the balancing of said bridge regulating the intensity of the light source to provide a unit'of illumination, means exterior of the casing for determining the intensity of a light source located at a distance from said casing and including a portable light receiver formed with a light opening and a socket for said photoelectric cell in communication with said opening, and coacting means in connection with said adapter, cell, light calibrating housing and light receiver for placing said cell in electrical relation with said cables when said cell is successively seated in said calibrating housing and said light receiver.

4. In a photometric apparatus, the combination with a Portable casing provided with an instrument panel, a photoelectric cell and adapter therefor, said adapter being removably associated with the casing; of a light metering system within said casing adapted to coact with said cell to indicate the response thereof, said system includ: ing a, panel-mounted light meter and a pair of electrically shielded cables extending from said panel, said cables constituting input and output leads for said system and terminating at their free ends in said adapter, means within said casing for calibrating said light meter in terms of units of illumination, said means including a calibrating housing disposed adjacent said panel and formed with a seat adapted to receive said photoelectric cell, a light source within said housing in operative relation with respect to said cell, means for regulating said light source to provide the illuminating efiect of a unit of illumination,

said panel, and formed with a seat adapted torecelve said photo I electric cell, a light source within said housing in operative relation with respect to said cell, means said means including a Wheatstone bridge network disposed within said casing and having said light source in one leg thereof, a panel-mounted calibrating meter across said bridge, a source of electrical energy, and means for balancing said .5

bridge, the balancing of said bridge regulating theintensity o! the light source to provide a unit of illumination, means exterior of the casing for determining the intensity of'a light source located at a distance from saidcasing, said means including a portabl light receiver formed with a light passage extending longitudinally of said receiver, one end oi said passage opening to the exterior of said receiver, the other terminating in a seat adapted to receive said photoelectric cell, light filtering means detachably associated with said receiver in intercepting relation with respect to its light passage, and coactlng means inconnection with said adapter, cell, light calibrating housing and light receiver for placing said cell in electrical relation with said cables when said cell is successively seated in said calibrating hous- I ing and said light receiver. 1

WILLIAM LmEN.

H. MACKENZIE.

' acaaaoe REFERENCES CITED The following references are of record in the flle of thispatent:

UNITED STATES PATENTS OTHER REFERENCES A Precision Direct Reading Densitometerby M. H. Sweet, Journal Optical Society of America 25 July 1942 page 400. (Copy in Division 7. 

